In recent years there has been growing interest in the use of organic semiconductors, including conjugated polymers, for various electronic applications such as organic solar cells.
Organic solar cells (OSC) have attracted much attention due to their potential low cost, high-throughput roll-to-roll production, flexibility and light weight. However, top-performance OSCs are all processed using halogenated solvents, which are environmentally hazardous and would thus require expensive mitigation to contain the hazards. Attempts to process OSCs from non-halogenated solvents lead to inferior performance.
One of the major problems holding back the wide-spread use of OSCs is that all high-efficiency (>10%) devices are currently processed from hazardous halogenated solvents such as chlorobenzene (CB), 1,2-dichlorobenzene and additives such as 1,8-diiodooctane (DIO). These solvents are harmful to people and the environment and current state-of-the-art OSCs are thus not truly environmentally friendly in their production processes. In addition, halogenated solvents do not exist in nature and their production requires relatively costly synthetic steps. Hydrocarbons are better choices of solvents for OSC production, as they are more environmentally friendly and readily available from petroleum. However, several reports have indicated that OSCs processed from non-chlorinated solvents generally result in significantly reduced PCE levels, and that the best OSCs are still processed from halogenated solvents. One reason for the much poorer performance is that state-of-the-art donor and/or acceptor materials for OSCs typically exhibit poor solubility in non-halogenated solvents, which results in a poor bulk-heterojunction (BHJ) morphology containing excessively large domains. On the other hand, controlling and optimizing BHJ morphology is one of the most important challenges for OSCs. Some morphology parameters (e.g., molecular orientation at or relative to the donor/acceptor (D/A) interface, polymer backbone orientation, and domain purity) are quite challenging to control. Therefore, new tools and insights are needed to improve the morphology and performance of OSCs.
One particular area of importance is the field of organic photovoltaics. Organic semiconductors (OSCs) have found use in OPV as they allow devices to be manufactured by solution-processing techniques such as spin casting and printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. State-of-the-art OPV cells consist of a blend film of a conjugated polymer and a fullerene derivative, which function as an electron donor and an electron acceptor, respectively. In order to achieve highly efficient OPVs, it is important to optimize both the polymer (donor) and fullerene (acceptor) components and to find a material combination yielding an optimal bulk heterojunction (BHJ) morphology that supports efficient exciton harvesting and charge transport properties. Recent improvements in the efficiencies of single-junction OPVs (efficiency ˜8-9%) have largely been due to the development of low-band-gap polymers, which are defined as polymers with an absorption onset of at least 750 nm or more and with a band-gap of 1.65 eV or less. For example, poly(3-hexylthiophene) (P3HT), a low-performance OPV polymer having a band-gap of ˜1.9 eV, is not considered to be a state-of-the-art polymer for OPVs.
The low-band-gap polymer materials and the polymer/fullerene formulations suggested in the prior art for use in OPVs still suffer from certain drawbacks. High-efficiency (>8%) OPVs can be achieved using many different low-band-gap polymers, but all are constrained to use hazardous halogenated solvents in the device processing. Replacing halogenated solvents with hydrocarbon solvents were previously found to decrease the OSC efficiency from 9.2% to 6-7%.
Polymer thieno[3,4-b]thiophene/benzodithiophene (PTB7), described in the prior art, can achieve 9.2% efficiency when halogenated solvents. The structure of PTB7 is

Using hydrocarbon solvents to process PTB7 resulted in OSC efficiency of about 6-7%.
The aim of the presently claimed subject matter is to provide environmentally friendly non-halogenated solvent-based formulations for organic solar cells without degrading the performance of OSCs.